Objection 3: The Causal Closure of Physics
A third and final objection I will consider is that chemical substances or structures cannot be strongly emergent because their being so would entail the possibility of downward causation. But downward causation is not possible, because the physical is causally closed. One must admit that the existence of strong emergence in chemistry is incompatible with the causal closure of the physical. Closure is widely assumed by philosophers, and is an essential part of the problem of causal exclusion (a philosophical pseudoproblem if ever there was one). Closure is hardly ever argued for however, honourable exceptions being the arguments offered by Brian McLaughlin 1992 and David Papineau 2002: 232-256, which I have responded to elsewhere (Hendry 2006a, 2010a, 2010b). This is not the place for a general review of evidence for closure, but I will conclude with the following argument.
Closure is a thesis that concerns the relationship of physics to everything else, so to find evidence for it we must look beyond the internal structure of physical theories, and see how they are applied to the special sciences. Of all the special sciences, chemistry has the closest relationship to physics, which as we have seen is embodied in two great scientific achievements. Firstly there is the twentieth-century discovery that chemical substances can be individuated, and their behaviour understood, in terms of their structures at the atomic scale. Secondly there is the fact that non-relativistic quantum mechanics provides a “theory of everything” for molecules, an all-encompassing framework within which to understand their dynamical behaviour. Yet neither of these facts entails closure. In short, chemistry is where one might expect to find the imperial ambitions of physics fully played out, if they are played out anywhere. It is where we might expect to see some evidence for closure. Yet, as I have argued above, strong emergence is a plausible interpretation of the evidence offered by the explanatory relationships between physics and chemistry, which must surely weaken the case for closure.
Acknowledgements: I am most grateful to Alex Carruth, John Heil, Tom Lancaster, Tom McLeish, Michele Paolini Paoletti and members of the Durham Emergence Project reading group for helpful comments on an earlier version of this paper. I am also grateful to the John Templeton Foundation for generously supporting the Durham Emergence Project itself.